All these apps help you read faster. If you are not yet reading at your speed limit.

What I mean by that is that they help you practice receiving data faster - which is definitely useful - I used a very different route, which I think gives you much greater improvement in speed, retention and understanding.

Reading individual words faster is fine, but many studies agree that taking in whole sentences, paragraphs, or even pages is more efficient in terms of overall understanding. So when trying to study, I tend to go down the route of speed reading the documentation first (approximately 1200 words per minute - with an accuracy rate of around 75%, which places it in the skimming reading style) and using this initial pass to then go back to challenging aspects and read them at around 500 wpm (at close to 100% accuracy).

I never used apps to learn this technique. Instead I was guided in paragraph reading intially, then whole-page reading, by my parents and through my school. In conjunction with work on wide comprehension of English and whatever subject you are reading, you can train your brain to do this very quickly - even now I tend to read a page of a novel in around 10 seconds, or an academic book in around 20.

A very high level view is described on the fourhourworkweek.com.

Be aware, though, that no app or technique will make you read faster. What makes you faster is practicing all the time! - Read books at every opportunity. I have books that I am currently reading in almost every room in the house, and a kindle for my commute, so I can read as often as possible.